1. Field of the Invention
This invention relates to apparatus for measuring the dielectric constant of materials and more specifically to such instruments which utilize the application of R-F or microwave energy to a transmission line, in or around which a substance under investigation is introduced to vary the propagation characteristics of the transmission line.
2. Description of the Prior Art
Sensing the dielectric constant of a substance or material provides useful information in a variety of applications. In addition to indicating that specific characteristic of a substance, the information can be indicative of moisture content or other transitory conditions. For example, a measurement of the dielectric constant of a material-in-process may indicate moisture content as data for process control. A variety of such applications exist.
In the past the dielectric constants of various substances, fluid or solid, have been measured by such techniques as introducing the substance between the plates of a capacitor and noting the resulting change in capacitance. In using such a technique, if the substance under test has electrical conductivity, resulting electrical currents may cause serious errors in the dielectric-constant measurement. To offset that problem it has been customary to use high-frequency signals across the capacitor. With high-frequency excitation, the capacitor becomes physically small, however, with the attendant result that only small areas may be observed in a single measurement and the sensitivity of the system tends to be poor.
It has also been suggested that shorted transmission lines, driven at high frequencies, may be used for measuring dielectric constants. In such systems the shorted line is immersed in the substance which is being tested. If the transmission line is coaxial in nature a slot is provided in the line to probe along the transmission line for the null point. By detecting the null location, the dielectric constant of the substance may be calculated. Unfortunately, the null which has been mentioned is very broad. Consequently, accurate measurement of the dielectric constant is rather difficult, except under controlled laboratory conditions.
Another approach for determining the constant has been to measure the impedance change in the transmission line which occurs as the dielectric is introduced. This approach eliminates the insensitive-null problem (described above) but the relative separation of the transmission line conductors and potential coupling of the transmission line to other conductors in the vicinity of the transmission line are likely to cause first-order errors in the measurement.
Still another approach, again utilizing transmission lines for the measurement of dielectric constants, has been to introduce a fast rise-time pulse into a transmission line with the dielectric substance under investigation being in the space between the conductors of the transmission line. The initial pulse and the return pulses (reflected from discontinuities) are then presented on an oscilloscope. By mathematical computation, the speed of propagation of the known-frequency signal, first in air and then in the dielectric material being tested, is determined. This system though used effectively in laboratories is not readily adaptable to direct and convenient readout of the dielectric constant for a substance or material under investigation.